Doppler ultrasound method and apparatus for monitoring blood flow

ABSTRACT

A pulse Doppler ultrasound system and associated methods are described for monitoring blood flow. A graphical information display includes simultaneously displayed depth-mode and spectrogram displays. The depth-mode display indicates the various positions along the ultrasound beam axis at which blood flow is detected. These positions are indicated as one or more colored regions, with the color indicating direction of blood flow and varying in intensity as a function of detected Doppler ultrasound signal amplitude or detected blood flow velocity. The depth-mode display also includes a pointer whose position may be selected by a user. The spectrogram displayed corresponds to the location identified by the pointer. Embolus detection and characterization are also provided.

TECHNICAL FIELD

The invention relates generally to medical monitoring and diagnosticprocedures and devices, and more particularly to a Doppler ultrasoundmethod and apparatus for monitoring blood flow.

BACKGROUND OF THE INVENTION

Doppler ultrasound has been used to measure blood flow velocity for manyyears. The well-known Doppler shift phenomenon provides that ultrasonicsignals reflected from moving targets will have a shift in frequencydirectly proportional to the target velocity component parallel to thedirection of the ultrasound beam. The frequency shift is the same forany object moving at a given velocity, whereas the amplitude of thedetected signal is a function of the acoustic reflectivity of the movingobject reflecting the ultrasound. Pulse Doppler ultrasound systemscommonly produce a spectrogram of the detected return signal frequency(i.e., velocity) as a function of time in a particular sample volume,with the spectrogram being used by a physician to determine blood flowcharacteristics of a patient.

Some Doppler ultrasound systems also have the capability to detect andcharacterize emboli flowing in the bloodstream. An example Dopplerultrasound system with embolus detection capability is described in U.S.Pat. No. 5,348,015, entitled “Method And Apparatus For UltrasonicallyDetecting, Counting, and/or Characterizing Emboli,” issued Sep. 20,1994, to Moehring et al., the disclosure of which is incorporated hereinby reference. Such ultrasound systems are advantageously used both fordiagnostic exams (to determine the presence and significance of vasculardisease or dysfunction) and during surgical interventions (to indicatesurgical manipulations that produce emboli or alter/interrupt bloodflow).

Typically, a user of ultrasound equipment finds it rather difficult toproperly orient and position an ultrasound transducer or probe on thepatient, as well as to select a depth along the ultrasound beamcorresponding to the desired location where blood flow is to bemonitored. This is particularly true in ultrasound applications such astranscranial Doppler imaging (TCD). The blood vessels most commonlyobserved with TCD are the middle, anterior, and posterior cerebralarteries, and the vertebral and basilar arteries. The Doppler transducermust be positioned so the ultrasound beam passes through the skull viathe temporal windows for the cerebral arteries, and via the foramenmagnum for the vertebral and basilar arteries. The user of theultrasound equipment may find it difficult to locate these particularwindows or to properly orient the ultrasound probe once the particularwindow is found.

A complicating factor in locating the ultrasound window is determinationof the proper depth at which the desired blood flow is located.Commonly, the user does not know if he is looking in the correctdirection at the wrong depth, the wrong direction at the right depth, orwhether the ultrasound window is too poor for appreciating blood flow atall. Proper location and orientation of the Doppler ultrasound probe,and the proper setting of depth parameters, is typically by trial anderror. Not only does this make the use of Doppler ultrasound equipmentquite inconvenient and difficult, it also creates a risk that thedesired sample volume may not be properly located, with thecorresponding diagnosis then being untenable or potentially improper.

SUMMARY OF THE INVENTION

In accordance with the invention, an information display is provided inconnection with Doppler ultrasound monitoring of blood flow. Theinformation display includes two simultaneously displayed graphicaldisplays. One graphical display is a blood locator display thatindicates locations along the axis of the ultrasound beam at which bloodflow is detected. The blood locator display includes a locationindicator, such as a pointer directed to a selected one of thelocations. The other graphical display is a spectrogram indicatingvelocities of monitored blood flow at the selected location. The bloodlocator display may include a color region corresponding with thelocations at which blood flow is detected. The intensity of the colormay vary as a function of detected ultrasound signal amplitude or as afunction of detected blood flow velocities.

The blood locator display allows a user to quickly locate blood flowalong the ultrasound beam axis. Using the blood locator display, thelocation of blood flow of particular interest can be further refined bythe user adjusting the aim of the ultrasound probe to produce greaterdisplayed intensity or spatial extent at the particular location ofinterest. The user may then select the position of the pointer to viewthe corresponding spectrogram. The user may also use the twosimultaneously displayed graphical displays to locate a particular bloodvessel by detecting temporal or other variations in the displays thatare consistent with the blood vessel.

A method of detecting and characterizing an embolus is also provided.Locations in which blood does and does not flow are determined, as wellas the direction of blood flow. A first ultrasound signal that may be anembolus is evaluated to determine if it corresponds with the locationswhere blood does and does not flow, as well as determining if itcorresponds with the direction and rate of blood flow. If the firstultrasound signal does not correspond with blood flow direction or rate,then it is identified as non-embolic. If the first ultrasound signaldoes correspond with blood flow direction, and if it corresponds solelywith locations where blood flows, then the first ultrasound signal isidentified as an embolic signal of a first type. If the first ultrasoundsignal does correspond with blood flow direction, and if it correspondsboth with locations where blood does and does not flow, then the firstultrasound signal is identified as an embolic signal of a second type.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed in color.Copies of this patent with color drawing(s) will be provided by thePatent and Trademark Office upon request and payment of the necessaryfee.

FIG. 1 is a graphical diagram depicting a first Doppler ultrasoundsystem display mode in accordance with an embodiment of the invention.

FIG. 2 is a graphical diagram depicting velocity and signal powerparameters used in preparation of the display mode of FIG. 1.

FIG. 3 is a graphical diagram depicting velocity and signal powerparameters used in preparation of an alternative embodiment of thedisplay mode of FIG. 1.

FIG. 4 shows the alternative embodiment of the display mode of FIG. 1 incolor.

FIG. 5 is a graphical diagram depicting the display mode of FIG. 4 andits use to identify the pulmonary artery.

FIG. 6 is a graphical diagram depicting a second Doppler ultrasoundsystem display mode in accordance with an embodiment of the invention.

FIG. 7 shows two views of the display mode of FIG. 6 in color.

FIG. 8 is the graphical diagram of the display mode shown in FIG. 1,further depicting and distinguishing embolic signals from artifactsignals.

FIG. 9 is a functional block diagram depicting a Doppler ultrasoundsystem in accordance with an embodiment of the invention.

FIGS. 10 and 11 are functional block diagrams depicting particulardetails of pulse Doppler signal processing circuitry included in theDoppler ultrasound system of FIG. 9.

FIGS. 12-16 are process flow charts depicting particular operationsperformed by the pulse Doppler signal processing circuitry of FIGS. 10and 11.

DETAILED DESCRIPTION OF THE INVENTION

The following describes a novel method and apparatus for providingDoppler ultrasound information to a user, such as in connection withmeasuring blood velocities to detect hemodynamically significantdeviations from normal values, and to assess blood flow for theoccurrence of microembolic signals. Certain details are set forth toprovide a sufficient understanding of the invention. However, it will beclear to one skilled in the art that the invention may be practicedwithout these particular details. In other instances, well-knowncircuits, control signals, timing protocols, and software operationshave not been shown in detail in order to avoid unnecessarily obscuringthe invention.

FIG. 1 is a graphical diagram depicting a first display mode of Dopplerultrasound information in accordance with an embodiment of theinvention. In this first display mode, referred to as an Aiming mode100, two distinct ultrasound displays are provided to the user. Adepth-mode display 102 depicts, with color, blood flow away from andtowards the ultrasound probe at various depths along the ultrasound beamaxis (vertical axis) as a function of time (horizontal axis).

The depth-mode display 102 includes colored regions 104 and 106. Region104 is generally colored red and depicts blood flow having a velocitycomponent directed towards the probe and in a specific depth range.Region 106 is generally colored blue and depicts blood flow having avelocity component away from the probe and in a specific depth range.The red and blue regions are not of uniform color, with the intensity ofred varying as a function of the detected intensity of the returnDoppler ultrasound signal. Those skilled in the art will understand thatsuch a display is similar to the conventional color M-mode display, inwhich variation in red and blue coloration is associated with variationin detected blood flow velocities. However, such M-mode displays havenot been used concurrently with a spectrogram and with the specificapplication of locating blood flow as an input to the spectrogram, fromwhich diagnostic decisions are made.

The Aiming mode 100 also includes a displayed spectrogram 108, with FIG.1 depicting a velocity envelope showing the characteristicsystolic-diastolic pattern. Like the depth-mode display 102, thespectrogram 108 includes data points (not shown) within the velocityenvelope that are colored in varying intensity as a function of thedetected intensity of the return ultrasound signal. The particularsample volume for which the spectrogram 108 applies is at a depthindicated in the depth-mode display 102 by a depth indicator or pointer109. In this way, a user of the ultrasound system can conveniently seeand select particular depths at which to measure the spectrogram 108.The depth-mode display 102 readily and conveniently provides theinformation concerning the range of appropriate depths at which ameaningful spectrogram may be obtained.

As described above, the color intensity of regions 104 and 106preferably vary as a function of the detected intensity of the returnultrasound signal. Referring to FIG. 2, a graphical diagram depicts howsuch color intensity is determined. In order to avoid display ofspurious information, signals that may be intense but low velocity (suchas due to tissue motion) are ignored and not displayed in the depth-modedisplay 102 of FIG. 1. This is referred to as clutter filtering and isdepicted in FIG. 2 as the threshold magnitude clutter cutoff limits forpositive and negative velocities. Similarly, low power signalsassociated with noise are also ignored and not displayed in thedepth-mode display 102 of FIG. 1. The user can determine the upper powerlimit for the color intensity mapping by selecting a power range value.Signals above a maximum power are then ignored—another clutter filteringwhich is especially helpful when monitoring blood flow in the cardiacenvironment. Those skilled in the art will appreciate that otherfiltering techniques may be employed to improve the depth-mode displayimage, including delta modulator or other suitably adapted filteringtechniques.

While the currently preferred embodiment of the depth-mode display 102employs color intensity mapping as a function of signal intensity, andfurther colored red or blue according to flow directions towards or awayfrom the probe, those skilled in the art will appreciate that colorintensity as a function of detected velocity may be employed instead. Insuch case, and as shown in FIG. 3, color intensity varies from theclutter cutoff magnitude to a maximum velocity magnitude, correspondingwith one-half the pulse repetition frequency (PRF). Detected signalshaving a power below the noise threshold or above the selected upperpower limit are ignored. FIG. 4 is a color figure that shows the Aimingmode display 100 in which the color intensity of the regions 104 and 106vary as a function of detected velocity. Both the depth-mode display 102and the spectrogram 108 are displayed relative to the same time axis,and the depth-mode display shows variation both in spatial extent and incolor intensity with the same periodicity as the heart beat. Thoseskilled in the art will also appreciate that instead of varying colorintensity solely as a function of signal amplitude or solely as afunction of velocity, one could advantageously vary color intensity as afunction of both signal amplitude and velocity.

The particularly depicted depth-mode display 102 shown in FIG. 1 shows asimplified display of a single, well-defined red region 104 and asingle, well-defined blue region 106. Those skilled in the art willappreciate that the number and characteristics of colored regions willvary depending on ultrasound probe placement and orientation. Indeed, acatalogue of characteristic depth-mode displays can be provided toassist the user in determining whether a particularly desired bloodvessel has, in fact, been located. Once the user finds thecharacteristic depth-mode display for the desired blood vessel, the usercan then conveniently determine the depth at which to measure thespectrogram 108.

The Aiming mode 100 enables the user to quickly position the ultrasoundprobe, such as adjacent to an ultrasound window through the skull sothat intracranial blood flow can be detected. Use of colorizedrepresentation of signal amplitude is particularly advantageous for thispurpose, since a strong signal is indicative of good probe location andorientation. The use of colorized representation of flow velocity maynot be as advantageous, except where blood flow velocities varysignificantly over blood vessel cross-section. However, when attemptingto monitor blood flow near appreciably moving tissue (e.g., cardiacmotion above clutter cutoff velocity), colorized representation of flowvelocities may be preferred.

Referring to FIG. 5, use of the Aiming mode 100 is shown in connectionwith identifying a particular blood vessel, such as the pulmonary arteryor femoral vein. In this case, a colorized representation of flowvelocity is advantageously used in the depth-mode display 102, becauseof the high variation in blood flow velocities in these particular bloodvessels. By observing the temporal variation in the depth-mode display102, and the corresponding spectrogram 108, a user can identify optimallocation of the pulmonary artery as follows: (1) the depth-mode displayof the pulmonary artery will be blue with the same periodicity as theheart beat; (2) the blue region will typically reside between 4 and 9 cmdepth; (3) along the time axis, the blue signal will be relativelyintense in the middle of systole, corresponding to peak velocity; and(4) the signal will have the largest vertical extent in the depth-modedisplay, indicating that the user has positioned the probe such that thelongest section of the pulmonary artery is aligned coincident with theultrasound beam during systole. The user can then adjust otherparameters, such as gate depth for the displayed spectrogram 108 andclutter filter parameters.

The Aiming mode 100 also indicates to the user where to set the depth ofthe pulse Doppler sample gate so that the spectrogram 108 will processDoppler shifts from desired blood flow signals. It is the spectrogram108 that is of primary clinical interest, allowing the user to observeand measure parameters associated with a particular blood flow andproviding information that might suggest hemodynamically significantdeviations in that blood flow. Along with the depth-mode display 102 andthe correspondingly selected spectrogram 108, the information displayedto a user also typically includes well-known numerical parametersassociated with the spectrogram, such as mean peak systolic velocity,mean end diastolic velocity, pulsatility index, and the relative changein mean peak systolic velocity over time. Those skilled in the art willappreciate that other parameters and displays may also be provided,including data provided by other monitoring devices, such as EKG- orEEG-related information.

The Aiming mode display 100 of FIG. 1 is particularly useful inpositioning and orienting the Doppler ultrasound probe, and in firstselecting a depth at which to measure the spectrogram 108. Followingprobe location and orientation and range gate selection, the user willtypically prefer to have an information display emphasizing theclinically valuable spectrogram 108. Referring to FIG. 6, a seconddisplay mode is shown that is referred to as a Spectral mode 110. Inthis mode, the spectrogram 108 occupies a larger display area. Insteadof the full depth-mode display 102, a compressed depth-mode display 112is provided. This compressed depth-mode display 112, on a shortened timescale, provides information concerning the depth of the sample volume atwhich the spectrogram 108 is taken, and the status of the blood flow inthat sample volume, towards or away from the probe. Thus, the user iscontinually informed concerning the desired sample volume depth andassociated blood flow. This allows for quick understanding andcompensation for any changes in the location of the desired samplevolume relative to the blood flow, such as due to probe motion. Thisalso allows a user of the ultrasound system to fine tune the samplevolume depth even while focusing primary attention on the clinicallyimportant spectrogram 108.

FIG. 7 shows two different views of the Spectral mode 110 in color. Inone view, the selected depth indicated by the pointer 109 in thecompressed depth-mode display 112 is not a location at which bloodflows, and consequently no there are no blood flow signals in thedisplayed spectrogram 108. In the other view, the selected depthindicated by the pointer 109 does coincide with blood flow, and acorresponding spectrogram 108 is displayed. In the particular embodimentshown in FIG. 7, the color intensity of the region 104 varies as afunction of detected velocity, and shows a characteristic colorvariation that may be associated with variation in blood velocity acrossblood vessel cross-section, a variation with depth in the alignment ofthe detected blood flow relative to the ultrasound beam axis, or both.

Those skilled in the art will appreciate the important advantagesprovided by the diagnostic information displays shown in FIGS. 1,4, 6,and 7. While the displayed spectrogram 108 is not itself new, today'spulse Doppler ultrasound systems that do not have B-mode capability lacka means for successfully and reliably locating and orienting anultrasound probe and determining an appropriate sample volume depth atwhich to detect the blood flow of interest. Also, while colorizedrepresentation of blood flow directions and speeds or signal amplitudeis well known in the art, such as in color M-mode displays, suchdisplays have not been used for the purpose of aiming ultrasound probesor in selecting particular sample volume depths for concurrentspectrogram analysis.

Referring to FIG. 8, the simultaneous presentation of the depth-modedisplay 102 and spectrogram 108 can also provide important informationfor detecting embolic signals and differentiating such signals fromnon-embolic artifacts. FIG. 8 depicts three events: A, B, and C. Inevent A, the depth-mode display 102 shows a particularly high intensitysignal having a non-vertical slope—i.e., a high-intensity signal thatoccurs at different depths at different times. In event A, the signalexists only within the boundary of one of the colored blood flow regions104 and 106. In the spectrogram 108, a particularly high intensitysignal is seen to have different velocities, bounded by the maximum flowvelocity, within a short temporal region within the heartbeat cycle.Event A is strong evidence of an embolus passing through a blood flowregion near the selected sample volume.

Event B is another likely candidate for an embolus. In this case, thehigh-intensity signal seen in the depth-mode display 102 isnon-vertical, but does not appear exclusively within a range of depthswhere blood is flowing. While this signal is strong enough and/or has along enough back scatter to appear outside the blood flow margin in thedepth-mode display 102, the spectrogram display 108 still shows thecharacteristic high intensity transient signal associated with anembolus. Event B is also evidence of an embolus, but likely an embolusdifferent in nature from that associated with event A. Although theparticular signal characteristics of various emboli have not yet beenfully explored in the depth-mode display, the distinction between eventsA and B is likely that of different embolus types. For example, event Amay be associated with a particulate embolus, whereas event B may beassociated with a gaseous embolus, with the different acousticproperties of a gas bubble causing the particularly long back scattersignal and the appearance of occurrence outside the demonstrated bloodflow margins.

Event C is an artifact, whether associated with probe motion or someother non-embolic event. Event C appears as a vertical line in thedepth-mode display 102, meaning that a high-intensity signal wasdetected at all depth locations at precisely the same time—acharacteristic associated with probe motion or other artifact.Similarly, the high-intensity signal displayed in the spectrogramdisplay 108 is a vertical line indicating a high-intensity signaldetected for a wide range of velocities (including both positive andnegative velocities and velocities in excess of the maximum blood flowvelocities) at precisely the same time. Event C then is readilycharacterized as an artifact signal, and not embolic in nature.

Those skilled in the art will appreciate that the simultaneous displayof the depth-mode display 102 and the spectrogram 108 provides not onlyconvenient means for locating the desired sample volume, but alsoprovides a particularly useful technique for distinguishing embolicsignals from artifact signals, and perhaps even for characterizingdifferent embolic signals. Such embolic detection and characterizationis easily observed by the operator, but can also be automaticallyperformed and recorded by the ultrasound apparatus.

Automatic embolus detection is provided by observing activity in two ormore sample gates within the blood flow at the same time. The systemdiscriminates between two different detection hypotheses:

(1) If the signal is embolic, then it will present itself in multiplesample gates over a succession of different times.

(2) If the signal is a probe motion artifact, then it will presentitself in multiple sample gates simultaneously.

These two hypotheses are mutually exclusive, and events that aredeclared embolic are done so after passing the “Basic IdentificationCriteria of Doppler Microembolic Signals” (see, for example, Stroke,vol. 26, p. 1123, 1995) and verifying that successive detection (bytime-series analysis or other suitable technique) of the embolic signalin different sample gates is done at different points in time, and thatthe time delay is consistent with the direction of blood flow. Thedifferentiation of embolic from artifact signals can be furtherconfirmed by also observing activity at one or more sample gates outsidethe blood flow.

FIG. 9 is a functional block diagram that depicts an ultrasound system150 in accordance with an embodiment of the invention. The ultrasoundsystem 150 produces the various display modes described above inconnection with FIGS. 1-8 on an integrated flat panel display 152 orother desired display format via a display interface connector 154. Thesignal processing core of the Doppler ultrasound system 150 is a masterpulse Doppler circuit 156 and a slave pulse Doppler circuit 158. TheDoppler probes 160 are coupled with other system components by a probeswitching circuit 162. The probe switching circuit 162 provides bothpresence-detect functionality and the ability to distinguish betweenvarious probes, such as by detecting encoding resistors used in probecables or by other conventional probe-type detection. By providing boththe master and slave pulse Doppler circuits 156 and 158, two separateultrasound probes 160 may be employed, thereby providing unilateral orbilateral ultrasound sensing capability (such as bilateral transcranialmeasurement of blood velocity in the basal arteries of the brain). Themaster and slave pulse Doppler circuits 156 and 158 receive theultrasound signals detected by the respective probes 160 and performsignal and data processing operations, as will be described in detailbelow. Data is then transmitted to a general purpose host computer 164that provides data storage and display. A suitable host computer 164 isa 200 MHz Pentium processor-based system having display, keyboard,internal hard disk, and external storage controllers, although any of avariety of suitably adapted computer systems may be employed.

The ultrasound system 150 also provides Doppler audio output signals viaaudio speakers 166, as well as via audio lines 168 for storage or foroutput via an alternative medium. The ultrasound system 150 alsoincludes a microphone 170 for receipt of audible information input bythe user. This information can then be output for external storage orplayback via a voice line 172. The user interfaces with the ultrasoundsystem 150 primarily via a keyboard or other remote input control unit174 coupled with the host computer 164.

FIGS. 10 and 11 depict particular details of the master and slave pulseDoppler circuits 156 and 158. To the extent FIGS. 10 and 11 depictsimilar circuit structures and interconnections, these will be describedonce with identical reference numbers used in both Figures. FIG. 10 alsodepicts details concerning the input and output of audio information toand from the ultrasound system 150 via the microphone 170, the speakers166, and the audio output lines 168 & 172, the operations of which arecontrolled by the master pulse Doppler circuit 156.

At the transducer input/output stage, each of the pulse Doppler circuits156 and 158 includes a transmit/receive switch circuit 175 operatingunder control of a timing and control circuit 176 (with the particulartiming of operations being controlled by the timing and control circuit176 of the master pulse Doppler circuit 156). The timing and controlcircuit 176 also controls operation of a transmit circuit 178 thatprovides the output drive signal causing the Doppler probes 160 (seeFIG. 9) to emit ultrasound. The timing and control circuit 176 alsocontrols an analog-to-digital converter circuit 180 coupled to thetransmit/receive switch 175 by a receiver circuit 182. The function andoperation of circuits 175-182 are well known to those skilled in the artand need not be described further.

The primary signal processing functions of the pulse Doppler circuits156 and 158 are performed by four digital signal processors P1-P4. P1 isat the front end and receives digitized transducer data from thereceiver 182 via the analog-to-digital converter circuit 180 and a databuffer circuit or FIFO 186. P4 is at the back end and performs higherlevel tasks such as final display preparation. A suitable digital signalprocessor for P1 is a Texas Instruments TMS320LC549 integer processor,and suitable digital signal processors for P2-P4 are Texas InstrumentsTMS320C31 floating point processors, although other digital signalprocessing circuits may be employed to perform substantially the samefunctions in accordance with the invention.

Received ultrasound signals are first processed by the digital signalprocessor P1 and then passed through the signal processing pipeline ofthe digital signal processors P2, P3, and P4. As described in detailbelow, the digital signal processor P1 constructs quadrature vectorsfrom the received digital data, performs filtering operations, andoutputs Doppler shift signals associated with 64 different range gatepositions. The digital signal processor P2 performs clutter cancellationat all gate depths. The digital signal processor P3 performs a varietyof calculations, including autocorrelation, phase, and powercalculations. P3 also provides preparation of the quadrature data forstereo audio output. The digital signal processor P4 performs most ofthe calculations associated with the spectrogram display, includingcomputation of the spectrogram envelope, systole detection, and alsoprepares final calculations associated with preparation of the Aimingdisplay.

Each of the digital signal processors P1-P4 is coupled with the hostcomputer 164 (see FIG. 9) via a host bus 187 and control data buffercircuitry, such as corresponding FIFOs 188(1)-188(4). This buffercircuitry allows initialization and program loading of the digitalsignal processors P1-P4, as well as other operational communicationsbetween the digital signal processors P1-P4 and the host computer. Eachof the digital signal processors P2-P4 is coupled with an associatedhigh-speed memory or SRAM 190(2)-190(4), which function as program anddata memories for the associated signal processors. In the particularlydepicted signal processing chain of FIG. 10 or 11, the digital signalprocessor P1 has sufficient internal memory, and no external program anddata memory need be provided. Transmission of data from one digitalsignal processor to the next is provided by intervening data buffer orFIFO circuitry 192(2)-192(4). The ultrasound data processed by thedigital signal processor P4 is provided to the host computer 164 viadata buffer circuitry such as a dual port SRAM 194.

Referring to FIG. 10, the digital signal processor P4 of the masterpulse Doppler circuit 156 also processes audio input via the microphone170, as well as controlling provision of the audio output signals to thespeakers 166 and audio output lines 168, 172. P4 controls the audiooutput signals by controlling operations of an audio control circuit196, which receives audio signals from both the master and the slavepulse Doppler circuits 156 and 158.

Referring to process flow charts shown in FIGS. 12-16, a detaileddescription will now be provided of the operations performed by of eachof the digital signal processors P1-P4 included in both the master andslave pulse Doppler circuits 156 and 158. Particular detailedcalculations and numerical information are provided to disclose acurrent embodiment of the invention, but those skilled in the art willappreciate that these details are exemplary and need not be included inother embodiments of the invention.

Referring to FIG. 12, the operations of digital signal processor P1 areas follows:

1. DIGITIZATION OF RAW DATA. Read A(1:N), a series of N 14-bit valuesfrom the input A/D. The values are converted at 4× the Doppler carrierfrequency (8 MHz), and commence synchronously with the start of thetransmit burst. N=1000 if the Doppler pulse repetition frequency (PRF)is 8 kHz, 1280 if the Doppler PRF is 6.25 kHz, and 1600 if the DopplerPRF is 5 kHz.

2. QUADRATURE VECTOR CONSTRUCTION. Construct two vectors with N/4 pointseach according to the following rules: Br(1:N/4)=A(1:4:N−3)−A(3:4:N−1),and Bi(1:N/4)=A(2:4:N−2)−A(4:4:N). Br and Bi are the digitallydemodulated quadrature Doppler values for a series of N/4 different gatedepths. The subtractions here remove DC bias from the data.

3. LOW-PASS FILTER COEFFICIENTS. Br and Bi contain frequencies up tocarrier/4, and need to be further filtered to remove noise outside thebandwidth of the Doppler transmit burst. The coefficients foraccomplishing this low-pass filtering are determined by a creating, withstandard digital filter design software such as MATLAB, an order 21low-pass FIR filter. The normalized cutoff of this filter is 2/(T*fs),where T is the time duration of the transmit burst, and fs is the samplerate of the data in Br and Bi (2 MHz). Call this filter C(1:21). Thecoefficients of this filter will vary as the transmit burst length ischanged by the user, and a bank of several different sets of filtercoefficients is accordingly stored to memory.

4. INDEX ARRAYS. Data from 64 range gate positions are to be processedand passed onto P2. For ease of graphical display, these range gatepositions are selected to be 1 mm apart. However, the quadrature vectorsBr and Bi do not contain elements that are spaced 1 mm apart—they are0.385 mm apart. Therefore, indices into the Br and Bi arrays are usedthat correspond to values falling closest to multiples of 1 mm, as ameans to decimating Br and Bi to 1 mm sampling increments. This is doneby having a prestored array of indices, D1(1:64), corresponding todepths 29:92 mm for 8 kHz PRF, and indices D2(1:64) and D3(1:64) withcorresponding or deeper depth ranges for 6.25 kHz and 5 kHz PRFs.

5. LOW-PASS FILTER AND DECIMATION OF QUADRATURE DATA. The Br and Biarrays are low-pass filtered and decimated to 64 gates by the followingrules (note <a,b> is the 32 bit accumulated integer dot product ofvectors a and b):

8 kHz PRF:

Er(j)=<C, Br(D1(j)+(−10:10))>

Ei(j)=<C, Bi(D1(j)+(−10:10))>, and j=1:64.

6.25 kHz PRF:

Er(j)=<C, Br(D2(j)+(−10:10))>

Ei(j)=<C, Bi(D2(j)+(−10:10))>, and j=1:64.

5 kHz PRF:

Er(j)=<C, Br(D3(j)+(−10:10))>

Ei(j)=<C, Bi(D3(j)+(−10:10))>, and j=1:64.

6. PASS RESULTS TO P2. Er and Ei, 128 values altogether, comprise theDoppler shift data for 1 pulse repetition period, over a set of 64different sample gates spaced approximately 1 mm apart. These arrays arepassed to P2 with each new transmit burst.

Referring to FIG. 13, the operations of digital signal processor P2 areas follows:

1. ACCUMULATE INPUT DATA. Collect a buffer of M Er and Ei vectors fromP1 over a period of 8 ms, into floating point matrices Fr and Fi. At thePRFs of [8,6.25,5]kHz, the matrices Fr and Fi will each containrespectively M=[64,50,40] vectors. The jth Er and Ei vectors at theirrespective destinations are denoted by Fr(1:64,j) and Fi(1:64,j) (theseare column vectors). The kth gate depth across the M collected vectorsis indexed by Fr(k,1:M) and Fi(k,1:M) (these are row vectors).

2. PRESERVATION OF RAW DATA AT “CHOSEN” GATE DEPTH. Reserve in separatebuffer the raw data at the user-chosen gate depth, k, at which theDoppler spectrogram is processed. This row vector data,Gr(1:M)=Fr(k,1:M) and Gi(1:M)=Fi(k,1:M), is passed forward to P3 andeventually to the host for recording purposes.

3. CLUTTER CANCELLATION. Apply a fourth order clutter cancellationfilter to each row of Fr and Fi. Hr(1:64,1:M) and Hi(1:64,1:M) are thedestination matrices of the filtered Fr(1:64,1:M) and Fi(1:64,1:M) data.Application of this filter with continuity requires maintaining statevariables and some previous Fr and Fi values. The coefficients of theclutter filter will vary depending on the user choice of [Low Boost, 100Hz, 200 Hz, 300 Hz, and High Boost]. These coefficients are available bytable lookup in processor RAM, given the user choice from the aboveoptions.

4. PASS RESULTS TO P3. Gr, Gi, Hr and Hi are passed to P3 for furtherprocessing.

Referring to FIG. 14, the operations of digital signal processor P3 areas follows:

1. ACCUMULATE INPUT DATA. Receive Gr, Gi, Hr and Hi from P2.

2. COMPUTE AUTOCORRELATION. Compute the first lag of the autocorrelationof the data at each gate over time. Use all M values at each gate inthis calculation. This will generate an array of 64 complex values, onefor each gate. For the kth gate depth, let P=Hr(k,1:M)+jHi(k,1:M). Thenthe first lag autocorrelation for this depth is AC(k)=<P(1:M−1),P(2:M)>.(Note that in a dot product of complex values, the second vector isconjugated. Also note that this and all dot products in P2, P3, or P4are floating point calculations.) In this manner, construct the complexvector AC(1:64).

3. COMPUTE PHASE FOR EACH AC VALUE. For each autocorrelation value, us afour quadrant arctangent lookup to determine the phase of the complexvalue. Specifically, ANGLE(k)=arctan(imag(AC(k)), real(AC(k))). TheANGLE(k) value is proportional to the mean flow velocity at the gatedepth k.

4. If embolus characterization (e.g., distinguishing a particle from abubble) capability is enabled, the method routes to a subroutinedescribed below in connection with FIG. 16.

5. COMPUTE POWER. Compute the signal power. Use all M values at eachgate in this calculation. This will generate an array of 64 real values,one for each gate. For the kth gate depth, again letP=Hr(k,1:M)+jHi(k,1:M). Then the power for this depth isPOWER(k)=<P(1:M),P(1:M)> (note that in a dot product of complex values,the second vector is conjugated). In this manner, construct the realvector POWER(1:64).

6. LOG COMPRESS POWER. Convert POWER to Decibels:POWERd(1:64)=10*log10(POWER(1:64)).

7. COMPUTE POWER TRACES FOR EMBOLUS DETECTION. For each of four presetgate depths (one being the user selected depth and the other three beingcorrespondingly calculated), compute power from a 60 point moving windowat M different positions of the window. Note that some history of thedata at the specific gate depths will be required to maintain thiscalculation without interruption from new data spilling in every 8 ms.Specifically, for gate n, POWER_TRACEn(i)=<Hr(n,i−59:i)+jHi(n,i−59:i),Hr(n,i−59:i)+jHi(n,i−59:i)>. Note 3 power traces are taken from theregion including the sample volume placed inside blood flow, while thefourth power trace is taken from a sample volume well outside the bloodflow.

8. COMPLEX BANDPASS FILTER FOR USE IN AUDIO OUTPUT PREPARATION. The minand max frequencies resulting from user specified spectral unwrapping ofthe spectrogram are used to determine a complex bandpass filter formaking the audio output sound congruent with what is shown on thespectrogram display. For example, if the unwrapping occurs at [−1,7]kHz,then the audio complex bandpass filter has edges at −1 kHz and +7 kHz. Abank of several sets of complex bandpass filter coefficients,corresponding to different unwrap ranges, is generated offline andplaced in memory. Each coefficient set corresponds to one of theunwrapping selections the user can make. Let the operative set of filtercoefficients be called UWa(1:O) and UWb(1:O), where O is the filterorder plus one.

9. AUDIO OUTPUT PREPARATION: RESAMPLE. At the gate depth selected by theuser, k, the Doppler shift signals are to be played out the audiospeakers. Before doing so, some prepping of the audio signals isimportant to match the user-selected spectral unwrapping. Resample theaudio signal Hr(k,1:M) and Hi(k,1:M) to twice the PRF by multiplexingthe respective arrays with zeros: Qr(k,1:2M)={Hr(k,1), 0, Hr(k,2), 0,Hr(k,3), 0, . . . , Hr(k,M), 0} and Qi(k,1:2M)={Hi(k,1), 0, Hi(k,2), 0,Hi(k,3), 0, . . . , Hi(k,M), 0}.

10. AUDIO OUTPUT PREPARATION: COMPLEX BANDPASS. Apply a complex bandpassfilter to Qr+jQi in order to remove the extra images introduced bymultiplexing the data with zeros:

R(n)=UWb(1)*Q(n)+UWb(2)*Q(n−1)+ . . .+UWb(O)*Q(n−O+1)−Uwa(2)*R(n−1)−Uwa(3)*R(n−2)− . . . −Uwa(O)*R(n−O+1)

where Q(k)=Qr(k)+jQi(k).

11. AUDIO OUTPUT PREPARATION: HILBERT TRANSFORM. The audio data in thesequence R(n) is in quadrature format and needs to be converted intostereo left and right for playing to the operator. This is done with aHilbert transform, and a 95 point transform, H(1:95), is used in thiswork—the coefficients can be obtained with formulas in the literature orstandard signal processing software such as MATLAB. The application ofthe Hilbert transform to a data sequence is done as an FIR filter.Construction of stereo separated signals RL and RR from R(n) is doneaccording to [RL=Hilbert(Rr)+Delay(Ri), RR=Hilbert(Rr)−Delay(Ri)] whereDelay is a (Nh+1)/2 step delay of the imaginary component of R, and Nhis the size of the Hilbert filter (95).

12. Pass Gr, Gi, ANGLE, POWERd, POWER_TRACE1, POWER_TRACE2,POWER_TRACE3, POWER_TRACE4, Rr, Ri, RL and RR to P4 for furtherprocessing.

Referring to FIG. 15, the operations of digital signal processor P4 areas follows:

1. ACCUMULATE INPUT DATA. Receive Gr, Gi, ANGLE, POWERd, POWER_TRACE1,POWER_TRACE2, POWER_TRACE3, POWER_TRACE4, Rr, Ri, RL and RR from P3.

2. CALCULATE SPECTROGRAM. Compute power spectrum via the followingsteps: a) Concatenate new points in the Rr+jRi sequence with old pointssuch that there are 128 points altogether, b) Multiply the 128 pointsequence against a 128 point Hanning window, c) Calculate P, the FFT ofthe 128 point sequence, d) Calculate Pd=10*log10(P), and e) FFTSHIFT thePd sequence such that DC is at its center.

3. ENVELOPE. Compute the maximum frequency follower or “envelope”function, E(j), which indicates the upper edge of the flow signals inthe spectrogram. This is an integer between 0 and 63, and is indexed byFFT calculation—i.e., for every spectral line calculation there is onevalue of E. Those skilled in the art will know of a variety ofalgorithms for making this calculation.

4. SYSTOLE DETECTION. Based on the maximum frequency follower, detectthe start of systole. When the systolic start has been determined, setSYSTOLE_FLAG=TRUE. Also calculate the end diastolic velocity value,VEND, the peak systolic velocity value, VPEAK, and the mean velocity,VMEAN.

5. AIMING DISPLAY PREPARATION. Prepare the Aiming display via thefollowing steps: a) Subtract the value of the “aim noise” parameter setby the user from the POWERd array: POWERd2=POWERd-aim_noise, b) multiplyPOWERd2 by a factor which is 64 (the number of color shades) divided bythe value of the “aim range” parameter set by theuser—POWERd3=POWERd2*64/aim_range, c) clip the resulting power data at 0on the low end and 63 on the high end—the values now correspond toentries in a 64-value red or blue color table, and place results inarray POWERd4, and d) multiply each of the power values by 1, 0 or −1,depending respectively on whether the associated ANGLE value is greaterthan the “filter cutoff parameter”, less in absolute value than thefilter cutoff parameter, or less than the negative of the filter cutoffparameter. This results in 64 values (one per gate depth) in the rangeof [−64,+63]. This modified aiming array, POWERd5, is ready to displayafter sending to the host computer.

6. SPECTROGRAM DISPLAY PREPARATION. Prepare the spectrogram display viathe following steps: a) Subtract the user-selected noise floor parameterfrom the array Pd—Pd2=Pd-spectral_noise, b) Rescale the spectral data tocontain 256 colors across the user-specified dynamicrange—Pd3=Pd2*256/spectral_range, c) truncate/clip the data to beinteger valued from 0 to 255—Pd4=min(255,floor(Pd3)), d) truncate thedata to 8 bits—Pd5=8 bit truncate(Pd4).

7. AUDIO OUTPUT. Send the arrays RR and RL, the right and left speakeraudio outputs, to the speakers via port writes.

8. INPUT MICROPHONE. Sample M values into vector MIC from the inputmicrophone port (M is # of transmit pulse repetitions within an 8 msperiod).

9. EMBOLUS DETECTION: BACKGROUND POWER IN POWER TRACES. For each of thefour power traces, POWER_TRACE1 . . . POWER_TRACE4, corresponding to thefour preset gate depths, compute a background power level. Recall thatPOWER_TRACEn contains M values, where M is # of transmit pulserepetitions within an 8 ms period). The background power value isobtained by a delta-follower for each trace, and is denoted by δ1, δ2,δ3, and d δ4.

δ1new=δ1old+Δ, where Δ=sign(δ1old-mean(POWER_TRACE1))*0.1 dB.

δ2new=δ2old+Δ, where Δ=sign(δ2old-mean(POWER_TRACE2))*0.1 dB.

δ3new=δ3old+Δ, where Δ=sign(δ3old-mean(POWER_TRACE3))*0.1 dB.

δ4new=δ4old+Δ, where Δ=sign(δ4old-mean(POWER_TRACE4))*0.1 dB.

This update in the background values is done once every M power values,or every 8 ms.

10. EMBOLUS DETECTION: PARABOLIC FIT. Apply a parabolic fit algorithm tothe power trace each gate and determine if an event is occurring duringthe 8 ms period. This fit must be applied to successive data windowsspaced apart by at most 1 ms. If the parabolic fit is concave down, andhas a peak that exceeds the background power for the gate depth by 6 dB(an arbitrary threshold), then an event is detected.

11. EMBOLUS DETECTION: TIME DETERMINATION. For any single-gate events,compute the exact time of the event by analyzing the power trace betweenthe −6 dB points on either side of the peak power of the event. Recordevent results and times so that current events may be compared to pastones.

12. EMBOLUS DETECTION: HIGH LEVEL CALCULATION. If the followingconditions are true, then set DETECTION=TRUE: a) at least two adjacentof three gates in vicinity of blood flow show events within a 40 ms timewindow, b) the gate outside the blood flow shows no detection, and c)the timing of events shows progression in the direction of blood flow(i.e., the embolus is not swimming upstream).

13. Pass Gr, Gi, POWERd5, Pd5, SYSTOLE_FLAG, VEND, VMEAN, VPEAK, MIC andDETECTION to host for further processing.

Referring to FIG. 16, the embolus characterization subroutine operationsof digital signal processor P3 are as follows:

4A. CALCULATE MATRIX ELEMENT MAGNITUDES of Hr+jHi:Hmag(1:64,1:M)=10*log10(Hr.{circumflex over ( )}2+Hi.{circumflex over ()}2).

4B. CALCULATE REFERENCE BACKGROUND POWER LEVEL Pb.Hmean=sum(sum(Hmag(1:64,1:M)))/(64*M). IF PbOLD>Hmean THEN Pb=PbOLD−0.1dB, ELSE Pb=PbOLD+0.1 dB. (This is a delta follower of the backgroundpower level).

4C. DETERMINATION OF R1 and R2, constants to be used incharacterization. T1=transmit burst length in microseconds. T2=pulserepetition period, in microseconds. We know a priori that elements ofHk(1:64) are attached to 1 mm increments in depth. Then R1=axialresolution in mm=c*T1/2, where c=1.54 mm/microsecond, and R2=2*R1. Forexample, a 20 cycle transmit burst at 2 MHz carrier frequency has R1=7.2mm, where R2=14.4 mm.

4D. DETECT EMBOLUS SIGNATURE by examining each column of Hmag(1:64,1:M)and determining longest contiguous segment of data such that eachelement in the contiguous segment is greater than Pb+XdB (X=3, e.g.).More specifically, let Hk(1:64)=Hmag(1:64,k). Locate longest sequencewithin Hk, demarcated by starting and ending indices Hk(i1:i2), suchthat Hk(i)>Pb+X if i1<=i<=i2. The length of this sequence is thendetermined by fitting the first three points of Hk(i1:i2) with aparabola, and finding the left most point on the abscissa, z1, where theparabola crosses the ordinate of Pb. If the parabola does not intersectthe line y=Pb, then z1=i1. Similarly, the last three points of Hk(i1:i2)are fitted with a parabola and z2 is located. If the parabola does notintersect the line y=Pb, then z2=i2. The length of Hk(i1:i2) is z2−z1.IF z2−z1<R1, then no embolus is present. If R1<z2−z1<R2, then aparticulate is present. If z2−z1>R2, then a bubble is present.

4E. Pass this information along to P4. If P4 agrees that an embolus isbeing detected, then attach the characterization information.

Those skilled in the art will appreciate that the invention may beaccomplished with circuits other than those particularly depicted anddescribed in connection with FIGS. 9-11. These figures represent justone of many possible implementations of a Doppler ultrasound system inaccordance with the invention. Likewise, the invention may beaccomplished using process steps other than those particularly depictedand described in connection with FIGS. 12-16.

Those skilled in the art will also understand that each of the circuitswhose functions and interconnections are described in connection withFIGS. 9-11 is of a type known in the art. Therefore, one skilled in theart will be readily able to adapt such circuits in the describedcombination to practice the invention. Particular details of thesecircuits are not critical to the invention, and a detailed descriptionof the internal circuit operation need not be provided. Similarly, eachone of the process steps described in connection with FIGS. 12-16 willbe understood by those skilled in the art, and may itself be a sequenceof operations that need not be described in detail in order for oneskilled in the art to practice the invention.

It will be appreciated that, although specific embodiments of theinvention have been described for purposes of illustration, variousmodifications may be made without deviating from the spirit and scope ofthe invention. For example, a user interface in accordance with thepresent invention may be provided by means other than a video display,such as a printer or other visual display device. Those skilled in theart will also appreciate that many of the advantages associated withthese circuits and processes described above may be provided by othercircuit configurations and processes. Accordingly, the invention is notlimited by the particular disclosure above, but instead the scope of theinvention is determined by the following claims.

What is claimed is:
 1. A visual display device for providing informationin connection with Doppler ultrasound monitoring of blood flow,comprising: a display controller structured to control a first graphicaldisplay and a second graphical display, the first graphical display toindicate as a function of time a plurality of locations along anultrasound beam axis at which blood flow is detected by varyingintensity as a function of detected Doppler ultrasound signal amplitudeat each of the locations, the second graphical display to indicatevelocities of monitored blood flow at a selected location included inthe first graphical display.
 2. The user interface of claim 1 whereinthe plurality of locations is a first plurality and wherein thegraphical display indicates a second plurality of locations along theultrasound beam axis at which blood flow is not detected.
 3. The userinterface of claim 1 wherein the first graphical display includes firstand second colors associated with blood flow in first and seconddirections, respectively, the first and second colors having intensitiesvarying as a function of detected Doppler ultrasound amplitude.
 4. Theuser interface of claim 1 wherein the first graphical display includes acolor region corresponding with the locations at which blood flow isdetected.
 5. The user interface of claim 1 wherein the first graphicaldisplay includes a color region corresponding with the locations atwhich blood flow is detected, the color having varying intensity as afunction of a detected Doppler ultrasound signal amplitude.
 6. The userinterface of claim 1 wherein the first graphical display includes acolor region corresponding with the locations at which blood flow isdetected, the color associated with detected blood flow direction andhaving varying intensity as a function of a detected Doppler ultrasoundsignal amplitude.
 7. The user interface of claim 1 wherein the firstgraphical display includes a color region corresponding with thelocations at which blood flow is detected, the color associated withdetected blood flow direction and having varying intensity as a functionof detected blood flow velocities and detected Doppler ultrasound signalamplitude.
 8. The user interface of claim 1 wherein the second graphicaldisplay is a spectrogram indicating the velocities of the monitoredblood flow at the selected location as a function of time.
 9. The userinterface of claim 1 wherein the first and second graphical displays areprovided simultaneously.
 10. The user interface of claim 1 wherein thedisplay controller further controls the first graphical display todisplay a location indicator identifying the selected location.
 11. Theuser interface of claim 10 wherein the first graphical display includesa color region corresponding with the locations at which blood flow isdetected, the location indicator being a pointer directed towards aposition within the color region, and wherein the second graphicaldisplay is a spectrogram indicating the velocities of the monitoredblood flow at the selected location as a function of time.
 12. Agraphical display for providing information in connection with Dopplerultrasound monitoring of blood flow, comprising: a display controller toprovide a blood locator display to depict as a function of time aplurality of locations along an ultrasound beam is at which blood flowis detected by varying intensity as a function of detected Dopplerultrasound amplitude at each of the locations, and to further provide aspectrogram to depict detected blood flow velocities as a function oftime at a selected one of the plurality of locations.
 13. The graphicaldisplay of claim 12 wherein the display controller further provides alocation indicator identifying the selected location.
 14. The graphicaldisplay of claim 13 wherein the location indicator is a pointer directedto the selected location depicted in the blood locator display.
 15. Thegraphical display of claim 12 wherein the blood locator display includesa color region corresponding to the depicted locations at which bloodflow is detected, the color having varying intensity as a function of adetected Doppler ultrasound signal amplitude.
 16. The graphical displayof claim 12 wherein the blood locator display includes a color regioncorresponding to the depicted locations at which blood flow is detected,the color associated with blood flow direction and having varyingintensity as a function of a detected Doppler ultrasound signalamplitude.
 17. The graphical display of claim 12 wherein the bloodlocator display includes a color region corresponding to the depictedlocations at which blood flow is detected, the color associated withblood flow direction and having varying intensity as a function of adetected blood flow velocities and detected Doppler ultrasoundamplitude.
 18. The graphical display of claim 12 wherein the bloodlocator display and the spectrogram are displayed simultaneously. 19.The graphical display of claim 12 wherein the plurality of locations isa first plurality and wherein the graphical display indicates a secondplurality of locations along the ultrasound beam axis at which bloodflow is not detected.
 20. A graphical display for a Doppler ultrasoundsystem to provide information in connection with Doppler ultrasoundmonitoring of blood flow, comprising: a display controller; a bloodlocator display coupled to the display controller and having a colorregion to depict as a function of time a plurality of locations along anultrasound beam axis at which blood flow is detected, the color of thecolor region having its intensity vary as a function of detected Dopplerultrasound amplitude; and a spectrogram coupled to the displaycontroller to depict detected blood flow velocities as a function oftime at a selected one of the plurality of locations.
 21. The graphicaldisplay of claim 20 wherein the color region has one of first and secondcolors corresponding with first and second detected blood flowdirections.
 22. The graphical display of claim of claim 20 wherein thecolor region has one of first and second colors corresponding with firstand second detected blood flow directions, the intensity of the colorvarying as a function of detected blood flow velocity and Dopplerultrasound signal amplitude.
 23. The graphical display of claim 20,further comprising a location indicator identifying the selectedlocation.
 24. The graphical display of claim 23 wherein the locationindicator is a pointer directed towards a position within the colorregion corresponding to the selected location.
 25. The graphical displayof claim 23 wherein the location indicator is a pointer directed towardsa position within the colored region corresponding to the selectedlocation, and wherein the colored region has one of first and secondcolors corresponding with first and second detected blood flowdirections, the intensity of the color varying as a function of detectedblood flow velocity and Doppler ultrasound signal amplitude.
 26. Thegraphical display of claim 20 wherein the blood locator display and thespectrogram are displayed simultaneously.
 27. The graphical display ofclaim 20 wherein the plurality of locations is a first plurality andwherein the graphical display indicates a second plurality of locationsalong the ultrasound beam axis at which blood flow is not detected. 28.A Doppler ultrasound system for processing ultrasound signals along anultrasound beam axis and for displaying information to a user concerningblood flow, comprising: an ultrasound transducer operable to detectultrasound signals and responsively produce corresponding electricalsignals; signal processing circuitry coupled with the transducer andoperable to receive the electrical signals and determine blood flowcharacteristics corresponding with the detected ultrasound signals; adisplay coupled with the signal processing circuitry and operable toprovide aiming graphical information indicating as a function of time aplurality of locations along the beam axis at which blood flow isdetected by varying intensity as a function of detected Dopplerultrasound am amplitude at each of the locations, and to further providespectral graphical information indicating blood flow velocities at aselected one of the locations indicated by the aiming graphicalinformation.
 29. The Doppler ultrasound system of claim 28 wherein theaiming graphical information includes a color region corresponding withthe locations at which blood flow is detected, the color associated withblood flow direction and having varying intensity as a function of oneof detected blood flow velocity and detected Doppler ultrasound signalstrength.
 30. The Doppler ultrasound system of claim 28 wherein thedisplay provides the aiming and spectral graphical informationsimultaneously.
 31. The Doppler ultrasound system of claim 28, whereinthe display further comprises a location indicator identifying theselected location.
 32. The Doppler ultrasound system of claim 31 whereinthe location indicator is a pointer directed towards a position withinthe display region corresponding to the selected location.
 33. TheDoppler ultrasound system claim 28 wherein the plurality of locations isa first plurality and wherein the graphical display indicates a secondplurality of locations along the ultrasound beam axis at which bloodflow is not detected.
 34. In a Doppler ultrasound system for processingultrasound signals along an ultrasound beam axis, a method of providinginformation to a user concerning blood flow, comprising: displayingfirst graphical information depicting as a function of time blood flowat a plurality of locations along the beam axis by varying intensity asa function of detected Doppler ultrasound amplitude at each of thelocations; and displaying second graphical information depicting bloodflow velocities at a selected one of the locations displayed by thefirst graphical information.
 35. The method of claim 34 whereindisplaying the first graphical information includes displaying alocation indicator directed to the selected location.
 36. The method ofclaim 34 wherein the selected location is determined by the user. 37.The method of claim 34 wherein displaying the first graphicalinformation includes displaying a color region corresponding withlocations where blood flow is detected.
 38. The method of claim 34wherein displaying the first graphical information includes displayingcolor having a varying intensity in correspondence with detected Dopplerultrasound signal amplitude.
 39. The method of claim 34 whereindisplaying the first graphical information includes displaying one offirst and second colors corresponding to blood flow in first and seconddirections, respectively.
 40. The method of claim 34 wherein displayingthe first graphical information includes displaying one of first andsecond colors corresponding to blood flow in first and seconddirections, respectively, and varying the intensity of the first andsecond colors in correspondence with detected blood flow velocities anddetected Doppler ultrasound amplitude.
 41. The method of claim 34wherein displaying the first graphical information includes displayingone of first and second colors corresponding to blood flow in first andsecond directions, respectively, and varying the intensity of the firstand second colors in correspondence with detected Doppler ultrasoundsignal amplitude.
 42. The method of claim 34 wherein the first andsecond graphical information are displayed simultaneously.
 43. Themethod of claim 34 wherein the plurality of locations is a firstplurality and wherein the graphical display indicates a second pluralityof locations along the ultrasound beam axis at which blood flow is notdetected.
 44. In a Doppler ultrasound system for processing ultrasoundsignals along an ultrasound beam axis, a method of detecting andcharacterizing emboli in blood flow, comprising: determining a firstplurality of locations along the beam axis in which blood flows anddetermining a second plurality of locations along the beam axis in whichblood does not flow; determining the direction in which the blood flowsat each of the first locations; and if a first ultrasound signal havingan intensity greater than a threshold intensity is received, then:determining if the first ultrasound signal corresponds with the firstlocations; determining if the first ultrasound signal corresponds withthe second locations; determining if the first ultrasound signalcorresponds with the determined direction and velocity of the bloodflow; if the first ultrasound signal does not correspond with thedetermined direction or velocity of the blood flow, then identifying thefirst ultrasound signal as a non-embolic signal; if the first ultrasoundsignal corresponds with the determined direction or velocity of theblood flow, and if the first ultrasound signal corresponds solely withthe first locations, then identifying the first ultrasound signal as anembolic signal of a first type; and if the first ultrasound signalcorresponds with the determined direction and velocity of the bloodflow, and if the first ultrasound signal corresponds both with the firstand second locations, then identifying the first ultrasound signal as anembolic signal of a second type.
 45. The method of claim 44, furthercomprising selecting one of the first locations, and wherein the firstultrasound signal is a signal corresponding with the selected location.46. The method of claim 44 wherein determining the first plurality oflocations in which blood flows includes displaying graphical informationhaving a color region corresponding to the first locations.
 47. Themethod of claim 44 wherein determining the first plurality of locationsin which blood flows includes displaying graphical information having acolor region corresponding to the first locations, and whereindetermining the direction in which the blood flows at each of the firstlocations includes selecting one of first and second colors for thecolor region, the first and second colors corresponding with first andsecond blood flow directions along the beam axis, respectively.
 48. Themethod of claim 44 wherein determining the first plurality of locationsin which blood flows includes displaying graphical information having acolor region corresponding to the first locations, and varying theintensity of the color as a function of detected Doppler ultrasoundsignal intensity, and wherein determining if the first ultrasound signalcorresponds with the first locations includes displaying a graphicalevent signal corresponding with the first ultrasound signal anddetermining if the graphical event signal is positioned within the colorregion.
 49. The method of claim 44 wherein determining the firstplurality of locations in which blood flows includes displayinggraphical information having a color region corresponding to the firstlocations, and varying the intensity of the color as a function ofdetected Doppler ultrasound signal intensity, and wherein determining ifthe first ultrasound signal corresponds with the determined directionand velocity of blood flow includes displaying a graphical event signalcorresponding with the first ultrasound signal and determining if thegraphical event signal is positioned in a predetermined orientationrelative to the color region.
 50. The method of claim 44 whereindetermining if the first ultrasound signal corresponds with thedetermined direction and velocity of blood flow includes determining ifthe first ultrasound signal corresponds with a velocity not exceeding amaximum velocity of the blood flow.
 51. In a Doppler ultrasound systemfor processing ultrasound signals along an ultrasound beam axis, amethod of locating a selected one of a plurality of blood vessels,comprising: determining a plurality of locations along the beam axis inwhich blood flows; displaying first graphical information depicting thelocations at which blood flow is detected; determining the velocity withwhich blood flows in each of the locations; selecting a first one of thelocations; displaying second graphical information depicting blood flowvelocities at the first location; detecting a temporal variation in thefirst graphical information; detecting a temporal variation in thesecond graphical information; and determining whether the detectedtemporal variations in the first and second graphical informationcorresponds with the selected blood vessel.
 52. The method of claim 51wherein displaying the first graphical information includes displaying acolor region corresponding with the locations at which blood flow isdetected.
 53. The method of claim 51 wherein displaying the firstgraphical information includes displaying a color region correspondingwith the locations at which blood flow is detected, and varying theintensity of the color as a function of detected Doppler ultrasoundsignal intensity.
 54. The method of claim 51 wherein displaying thefirst graphical information includes displaying a color regioncorresponding with the locations at which blood flow is detected, andvarying the intensity of the color as a function of the determined bloodflow velocities.
 55. The method of claim 51 wherein displaying the firstgraphical information includes displaying a color region correspondingwith the locations at which blood flow is detected, and whereindetecting a temporal variation in the first graphical informationincludes detecting a temporal variation in the size of the color region.56. The method of claim 51 wherein displaying the first graphicalinformation includes displaying a color region corresponding with thelocations at which blood flow is detected, and varying the intensity ofthe color as a function of the determined blood flow velocities, andwherein detecting a temporal variation in the first graphicalinformation includes detecting a temporal variation in the colorintensity of the color region.
 57. A computer readable medium whosecontents configure a computer system to provide information to a userconcerning blood flow detected by processing Doppler ultrasound signalsalong an ultrasound beam axis, comprising: displaying first graphicalinformation depicting as a function of time blood flow at a plurality oflocations along the beam axis by varying intensity as a function ofdetected Doppler ultrasound amplitude at each of the locations; anddisplaying second graphical information depicting blood flow velocitiesat a selected one of the plurality of locations.
 58. The computerreadable medium of claim 57 wherein displaying the first graphicalinformation includes displaying a location indicator directed to theselected location.
 59. The computer readable medium of claim 57 whereinthe selected location is determined by a user of the computer system.60. The computer readable medium of claim 57 wherein displaying thefirst graphical information includes displaying a color regioncorresponding with locations where blood flow is detected.
 61. Thecomputer readable medium of claim 57 wherein displaying the firstgraphical information includes displaying color having a varyingintensity in correspondence with detected Doppler ultrasound signalamplitude.
 62. The computer readable medium of claim 57 whereindisplaying the first graphical information includes displaying one offirst and second colors corresponding to blood flow in first and seconddirections, respectively.
 63. The computer readable medium of claim 57wherein displaying the first graphical information includes displayingone of first and second colors corresponding to blood flow in first andsecond directions, respectively, and varying the intensity of the firstand second colors in correspondence with detected blood flow velocitiesand detected Doppler ultrasound amplitude.
 64. The computer readablemedium of claim 57 wherein displaying the first graphical informationincludes displaying one of first and second colors corresponding toblood flow in first and second directions, respectively, and varying theintensity of the first and second colors in correspondence with detectedDoppler ultrasound signal amplitude.
 65. The computer readable medium ofclaim 57 that further configure the computer system to display the firstand second graphical information simultaneously.
 66. The user interfaceof claim 57 wherein the plurality of locations is a first plurality andwherein the graphical display indicates a second plurality of locationsalong the ultrasound beam axis at which blood flow is not detected.